The first exoplanet has been discovered in 1995 by Mayor and Queloz. Since that moment, more than one hundred and fifty planets have been detected within ten years. All of these planets were found by indirect methods, which means that only some effects were detected that the planet has on its star and not direct radiation from the planet.
Direct detection of an Earth-like exoplanets is not an easy task. Indeed, if our solar system was seen from a distance of 10 pc, the angular separation between Earth and Sun would be equal to 0.5 mrad and the brightness contrast between the star and the planet would be, in the best case, 106.
Nulling interferometry seems a quite promising technique up to now. It consists in observing a star-planet system with an array of telescopes, and then combining the light from these telescopes in such a way that, simultaneously, destructive interference occurs for the star light and (partially) constructive interference for the planet light. The ratio between the intensities corresponding to constructive and destructive interference is called the rejection ratio. To be able to detect a planet, this ratio is preferably of the order of at least 106.
Another major difficulty is that this rejection ratio is preferably achieved in a wide spectral band (6-18 mm or even wider). This wide band is required to obtain spectral information from the planet and to optimally exploit the photon flux from the planet.
To reach this high rejection ratio in a wide spectral band, most current nulling interferometers use a (achromatic) phase shifter.
However, the phase shifter embodiments are problematic since for more than two beams they typically result in asymmetric setups where multiple incoming beams are retarded in different ways so as to shift the phases thereof. These setups are generally difficult to handle.